Optical fibers are useful for many applications in modern communications systems. A typical optical communications system comprises a transmitter of optical signals, a length of transmission optical fiber coupled to the source, and a receiver coupled to the fiber for receiving the signals. One or more amplifying systems may be disposed along the fiber for amplifying the transmitted signal. Filters and attenuators are required in these systems to change the power levels of various signals.
Basically, an optical fiber is a strand of waveguiding glass comprising an inner core having a certain index of refraction and an outer cladding peripherally surrounding the core. The cladding has a lower index of refraction than the core. As long as the refractive index of the core exceeds that of the cladding, a light beam can be guided along the length of the core by total internal reflection. Since the light is confined mostly in the core region, the ability to externally effect propagation behavior of the light is significantly limited. With conventional fibers, to change the propagation behavior of light in the core, one is essentially limited to the application of strain and/or temperature changes to the fiber.
Optical fiber gratings are important elements for controlling light within fibers. A fiber grating typically comprises a length of fiber including a plurality of substantially equally spaced optical grating elements such as index perturbations, slits or grooves. Such gratings include long period gratings and Bragg gratings.
A long period grating couples optical power between two copropagating modes with very low back reflections. It typically comprises a length of optical waveguide wherein the refractive index perturbations are spaced by a periodic distance Λ which is large compared to the wavelength λ of the transmitted light. In contrast with Bragg gratings, long-period gratings use a periodic spacing Λ which is typically at least 10 times larger than the transmitted wavelength, i.e. Λ≧10λ. Typically Λ is in the range 15–1500 micrometers, and the width of a perturbation is in the range ⅕ Λ to ⅘ Λ. In some applications, such as chirped gratings, the spacing Λ can vary along the length of the grating. Long-period gratings are particularly usefull in optical communication systems for equalizing amplifier gain at different wavelengths. See, for example, U.S. Pat. No. 5,430,817 issued to A. M. Vengsarkar on Jul. 4, 1995.
A fiber Bragg grating comprises a length of optical fiber including a plurality of perturbations in the index of refraction. These perturbations selectively reflect light of wavelength λ equal to twice the spacing Λ′ between successive perturbations times the effective refractive index, i.e. λ=2neffΛ′, where λ is the vacuum wavelength and neff is the effective reactive index of the propagating mode. The remaining wavelengths pass essentially unimpeded. Bragg gratings have found use in a variety of applications including filtering, adding and dropping signal channels, stabilization of semiconductor lasers, reflection of fiber amplifier pump energy, and compensation for waveguide dispersion.
Many potential applications require optical gratings having characteristics which are tunable. A tunable long period grating can provide dynamic gain compensation. Tunable Bragg gratings can permit dynamic control of which wavelength will pass through the grating and which will be reflected or diverted.
As may be appreciated, those concerned with the development of optical communications systems continually search for new components and fiber designs. As optical communications systems become more advanced, there is growing interest in new methods and devices for modulating, filtering, attenuating and switching wavelength channels. The instant invention provides a new structure for a tunable optical fiber devices including tunable fiber gratings.